1. Field of the Invention
The present invention relates to a workpiece dividing method using a laser beam, and more particularly to a method of dividing a workpiece such as a semiconductor wafer or a light-emitting device wafer with a laser beam.
2. Description of the Related Art
There is known a method of dividing a workpiece such as a semiconductor wafer or a light-emitting device wafer along streets (dividing lines) with a laser beam (see, for example, Japanese Patent No. 3408805). According to the dividing method using the laser beam, a pulsed laser beam is applied to a wafer while being focused on a point within the wafer, continuously forming a modified layer within the wafer along streets. As the wafer has its mechanical strength lowered at the modified layer, the wafer can be divided along the streets when subjected to external forces.
Plate-like workpieces such as wafers or the like are liable to suffer thickness variations due to warpage, undulation, etc. When such a workpiece is to be divided by a laser beam, a uniform modified layer may not be formed in the workpiece at a prescribed depth on account of thickness variations thereof. As a solution to the problem, there is known a laser machining apparatus for detecting surface displacements (surface irregularities) on streets of a workpiece and controlling the focused spot of a laser beam to follow the detected surface displacements (see, for example, Japanese Patent Laid-Open No. 2010-046703).
According to Japanese Patent Laid-Open No. 2010-046703, the laser machining apparatus applies a detecting laser beam to the surface of a workpiece that is held by a holding table, and measures changes in the light intensity of a laser beam reflected from the surface of the workpiece to determine surface displacements of the workpiece. Based on the detected surface displacements of the workpiece, the laser machining apparatus focuses a machining laser beam while adjusting the focus thereof to form a modified layer in the workpiece at a prescribed depth from the surface of the workpiece.